THIS INVENTION relates to the production of hydrocarbons. It relates in particular to a process and a reactor for producing hydrocarbons.
According to a first aspect of the invention, there is provided a process for producing hydrocarbons, which process comprises
allowing reactants forming part of a reaction medium in a reaction zone, to react at reaction conditions so as to form primary hydrocarbon products, with water being formed as a by-product; and
allowing by-product water, on formation thereof under the reaction conditions, to permeate through a membrane capable of selectively removing water from the reaction medium, thereby to be separated from the reaction medium.
The reactants may be in gaseous form. The reactants may, in particular, comprise carbon monoxide and hydrogen, with a particulate Fischer-Tropsch catalyst also forming part of the reaction medium. The reaction conditions will then be selected such that the carbon monoxide and hydrogen react in the presence of the Fischer-Tropsch catalyst, to produce, as the primary hydrocarbon products, liquid Fischer-Tropsch derived hydrocarbon product(s), and/or gaseous Fischer-Tropsch derived product(s), in accordance with a simplified Fischer-Tropsch reaction equation (1):
CO+(1+x)H2xe2x86x92CH2x+H2O xe2x80x83xe2x80x83(1) 
The process may include continuously feeding a synthesis gas comprising the carbon monoxide and hydrogen into the reaction zone where the Fischer-Tropsch reaction takes place, while thus maintaining the reaction zone at typical Fischer-Tropsch operating conditions.
The reaction zone may be provided by a slurry bed reactor or by a fluidized bed reactor. In the case of a slurry bed reactor, the slurry bed thereof will comprise liquid hydrocarbon products, gaseous hydrocarbon products, water, synthesis gas, and catalyst particles. In the case of a fluidized bed reactor, the fluidized bed thereof will comprise gaseous hydrocarbon products, synthesis gas, water, and catalyst particles. The membrane is thus located in the slurry or fluidized bed so that by-product water can be removed as it is formed in accordance with equation (1). The reaction medium thus comprises, in the case of the slurry bed reactor, the liquid hydrocarbon products, the gaseous hydrocarbon products, water, synthesis gas and the catalyst particles. In the case of the fluidized bed reactor, the reaction medium comprises the gaseous hydrocarbon products, synthesis gas, water, and the catalyst particles.
The synthesis gas thus enters the reaction zone of the reactor at a low level below, or at the bottom of, the slurry or fluidized bed. The liquid hydrocarbon products are withdrawn from the bed through a liquid product outlet, in the case of the slurry bed reactor, while the gaseous hydrocarbon products are withdrawn from the top of the reaction zone, in the case of the slurry and fluidized bed reactors.
The Fischer-Tropsch catalyst may be iron-based, cobalt-based, or iron- and cobalt-based. It is, as hereinbefore described, in particulate form.
The temperature in the reaction zone may be between 160xc2x0 C. and 380xc2x0 C., typically about 200xc2x0 C. to 250xc2x0 C. in a slurry bed reactor, and typically about 300xc2x0 C. to 360xc2x0 C. in a fluidized bed reactor. The pressure may be between 1800 kPa(a) and 5000 kPa(a), typically about 2000 kPa(a) to 4500 kPa(a).
Water is thus, as is evident from equation (1), one of the main products of the Fischer-Tropsch reaction, in addition to the primary products, ie in addition to the hydrocarbons.
It is also evident from equation (1) that the higher the per pass (H2+CO) conversion, the higher the partial pressure of water inside the reactor.
Although it is a product from the Fischer-Tropsch reaction, water has unfavourable or negative effects on the Fischer-Tropsch catalyst.
These unfavourable or negative effects of water, in the case of an iron-based Fischer-Tropsch catalyst, include the water decreasing the Fischer-Tropsch reaction rate; the water reoxidizing, and hence deactivating, the catalyst; and, in the case of a precipitated iron-based catalyst, the water lowering the mechanical strength of the catalyst particles. In the case of a cobalt-based catalyst, reaction water also causes reoxidation, and hence deactivation, of the catalyst; and, in the case of a precipitated cobalt-based catalyst, the water lowers the mechanical strength of the catalyst particles.
These unfavourable effects are thus avoided or at least reduced by removing by-product or reaction water from the reaction medium by means of the membrane, in accordance with the invention.
Additionally, in a slurry or fluidized bed Fischer-Tropsch reactor, alongside the (principle) Fischer-Tropsch reaction in accordance with equation (1), there occurs a reversible water-gas shift (WGS) reaction, in accordance with equation (2):
CO+H2O⇄CO2+H2xcex94H=xe2x88x9241.1 kJ/mol xe2x80x83xe2x80x83(2) 
According to the present invention, water is continuously removed from the reaction medium as it is produced by the Fischer-Tropsch reaction. Accordingly, the water partial pressure decrease has a direct influence on the WGS reaction, in a sense that, in equation (2) the equilibrium shifts to the left, following the well-known Le Chatelier""s principle.
This results in some unexpected advantages, viz undesired CO consumption in the WGS reaction is thus inhibited so that more CO is available for reaction in the Fischer-Tropsch reaction, thus improving the carbon (monoxide) consumption efficiency of the process, the equilibrium shift to the left, in equation (2), results in an overall decrease in the H2O/CO ratio in the reaction medium, which translates into an increase in the selectivity potential of the Fischer-Tropsch reaction towards desired unsaturated hydrocarbons and oxygenated chemicals, eg carbonyls, rather than paraffins; and a lower H2/CO ratio results in an increase in the xcex1-value (the chain-grown probability factor).
The membrane may be supported by a water-permeable, eg porous, support such that the membrane has a water inlet side or surface and a water outlet side or surface, with the by-product water thus entering the membrane through its water inlet side or surface, permeating through the membrane, and exiting the membrane through its water outlet side or surface. The support and the membrane thus form a water separation device.
The process may include passing an inert sweep gas along the support in proximity to the water outlet side of the membrane, to entrain water which permeates through the membrane, thereby to provide a driving force for water permeation through the membrane.
The separation device may be of any suitable shape or configuration, eg it may be of tubular shape, and may then be of elongate form, U-shaped, or of an other suitable form or configuration. The separation device may be oriented at any suitable inclination, eg the separation device, or the major components thereof such as its limbs when it is U-shaped, may be located horizontally, vertically, or at an angle to the vertical.
When the support is of tubular form, the membrane may be provided on the inner or on the outer surface of the tubular support, with the sweep gas passing through the inside of the membrane. Thus, the sweep gas may enter the reaction zone through a conduit connected to the inside of the tubular support at or near one end thereof, pass through the support, and exit the reaction zone through another conduit leading from the support at or near another end thereof, out of the reaction zone. In this fashion, the sweep gas is thus not exposed to the reaction medium. The sweep gas may be any suitable inert gas such as nitrogen, synthesis gas, or the like.
While the membrane can, at least in principle, be of any suitable material capable of selectively removing water from the reaction medium such as of polymeric material, it may, in particular, be of zeolitic material, ie it may be zeolite-based.
Any suitable zeolite capable of selectively removing water from the reaction medium may, at least in principle, be used for the zeolite based or zeolitic membrane. Thus, the zeolite may be selected from mordenite, ZSM-5, zeolite A, or chabazite; however, mordenite is preferred.
The support may be of any suitable water-permeable material having sufficient strength to support the membrane, such as a porous metal eg stainless steel, a ceramic eg alpha or gamma alumina, a multichannel support, or the like; however, porous stainless steel is preferred.
The thickness of the membrane will depend primarily on the preparation procedure of the separation device, and on the pore size of the support employed. Typically, the membrane thickness may be in the range of 5 to 30 microns, ie micrometers. It is believed that the membrane thickness will affect both the permeation flux and the probability of defects occurring in the membrane.
The support may have a thickness of from 1 to 2 mm, and may have pores in the range of 5 nanometers to 500 nanometers. Thus, in one embodiment of the invention, the support may be of porous stainless steel having pores of about 500 nanometers, eg as obtainable from Mott Metallurgical Co. In another embodiment of the invention, the support may be of porous gamma-alumina having pores of about 5 nanometers, eg as obtainable from Societe des Ceramiques Techniques. In yet another embodiment of the invention, the support may be of porous gamma-alumina having pores of about 60 nanometers, eg as obtainable from Inocermic. In still another embodiment of the invention, the support may be of porous alpha-alumina having pores of about 200 nanometers, eg as obtainable from Societe des Ceramiques Techniques.
The Applicant has surprisingly found that the water permeation rate as well as the selectivity for the removal of water strongly depend on the temperature and on the partial pressure of water in the reaction zone. High selectivities and high water permeation fluxes occur at the high reaction temperatures and high water partial pressures. Selectivity also increases with an increase in the molecular weight of the hydrocarbons that are formed.
The invention has the advantages of improving the productivity of the Fischer-Tropsch process, and the lifetime of iron and cobalt based Fischer-Tropsch catalysts. The zeolite based membrane that is used in this invention is capable of selectively extracting the reaction water from the Fischer-Tropsch reaction medium with high (water/species i) separation factors, with species i being, for example, hydrocarbons, CO, CO2 or H2. This occurs even under relatively low partial pressures of water.
According to a second aspect of the invention, there is provided a reactor for producing hydrocarbons, which comprises
a reactor vessel having a catalyst bed zone which, in use, will contain a slurry or fluidized bed of catalyst particles suspended or fluidized in liquid and/or gaseous hydrocarbon product;
a gas inlet in the vessel at a low level within the catalyst bed zone, for introducing gaseous reactants into the vessel;
a gas outlet in a head space of the vessel above the catalyst bed zone, for withdrawing unreacted gaseous reactants and gaseous products from the vessel; and
a membrane capable of selectively removing water from a reaction medium, located in the catalyst bed zone so that any by-product water which is formed in the reaction zone on reaction of the gaseous reactants to form the liquid and/or gaseous hydrocarbon product, can permeate through the membrane.
As hereinbefore described, the membrane may be supported by a water-permeable support such that the membrane has a water inlet side or surface and a water outlet side or surface, with the by-product water thus, in use, entering the membrane through its water inlet side; permeating through the membrane, and exiting the membrane through its water outlet side. The support and the membrane may thus form a water separation device, as hereinbefore described.
The support may, in particular, be of tubular form having a central passageway, with the membrane being provided on the inner or the outer surface of the tubular support. A sweep gas feed conduit may be connected to one end of the central passageway of the tubular membrane, for feeding an inert sweep gas into and along the central passageway of the membrane, with a sweep gas withdrawal conduit leading from the other end of the membrane central passageway.
The reactor may, in particular, be a slurry or fluidized bed reactor as hereinbefore described. The reactor may still more particularly be used for producing hydrocarbons by means of a Fischer-Tropsch reaction, using a Fischer-Tropsch catalyst, as hereinbefore described.